The present invention relates to a device for transmitting optical signals, particularly via interfaces between components mobile for rotation or translation.
In many applications with optical data transmission using optical waveguides, a transmission via rotating interfaces is also required. To this end, various appropriate single-channel transmission systems have become known in the prior art. As the data rates increase, the number of channels to be transmitted is also increased. Hence the fields of application for multi-channel transmission systems have gone through a strong expansion.
When cable drums with optical waveguides in the cable are used, for example, in which data must be transmitted via the optical waveguides while the cable is reeled off or wound onto the drum, an optical multi-channel transmitter is necessary. Such cable drums are employed, for instance, in remotely operated vehicles (ROV) for movement on land or at sea (bomb deactivation drone, diving robots in offshore industry) or as sonar towed vehicles trailing behind ships for surveying the sea floor. Examples of other fields of application for multi-channel transmitter are rotatable remote prospecting and reconnaissance instruments for civil, scientific and military applications, which generate high data streams (radar, IR/visual, etc.), such as camera heads under helicopters, unmanned aerial vehicles (UAV), large-scale telescopes, satellites.
The on-board application in ships and aircraft requires a sturdy design (shocks and vibrations), especially as far as the adjustment stability is concerned (which is obviously easier to realize in a small number of small components than in a great number of large ones). In the case of aircraft, particularly in drone missiles, a low weight and small dimensions constitute an additional decisive criterion that is hardly satisfied by the solutions so far known.
Numerous approaches to solutions are known for multi-channel rotary transmission. The essay by Speer and Koch “The diversity of fibre-optic rotary connectors” in: SPIE vol. 839, pp. 122–129, provides a sound survey.
The approaches to solutions can be classified as follows on principle:
1. Concentric fiber bundles in which the light of the respective channel to be transmitted is distributed among concentric (hollow) cylindrical optical waveguide arrays that can be rotated relative to each other at the interface as described in U.S. Pat. No. 4,027,945.
The essential disadvantage of this known array resides in the fact that the optical waveguides require increasing diameters as the respective channel is located farther to the outside. This system can be very well realized, for example, with synthetic optical waveguides. A transmission with single-mode fibers is practically precluded because these fibers can hardly be mounted in a sufficient number with a sufficient precision as a result of their extremely small core diameter. As the fiber diameter increases, the dispersion increases as well and hence the bandwidth or data rates that can be transmitted are reduced. This system does therefore not satisfy the demands on advanced data transmission systems requiring transmission in the Gbit/s range.
2. Coaxial imaging arrays are substantially based on the fact that a fixed (focal) point on the other side of the interface is associated with an emitting element rotating in an annular zone. The imaging function is implemented by an appropriate optical element such as a lens with different focal lengths for different annular zones (published in the laid-open German patent application DE 32 07 469 A1) or a holographic-optical element (published in the laid-open German patent application DE 197 80 642 A1).
The disadvantage of the first-mentioned known approach, i.e. the situation that all receiving fibers are disposed on the optical axis (OA) and that hence shading occurs, is avoided in the holographic-optical elements (HOE). However, the production of the HOE elements is presently still expensive when high efficiency ratios are required, and, due to the smaller angles of deviation in the preferably employed binary HOE elements, large structural dimensions are involved. For single-mode fibers, HOE elements can presently be used only with high losses. Moreover, this method can be applied in a unidirectional mode only. Advanced high-speed bus systems, however, principally require a bi-directional communication.
3. Imaging arrays with rotation compensation. Here, an optical element such as a Dove prism (described in U.S. Pat. No. 3,428,821) or a “de-rotation plate” (U.S. Pat. No. 4,258,976) rotating at precisely half the rotational speed compensates for the rotation. Hence, an array of the faces of the incoming fibers is projected onto the associated respective outgoing fibers.
The support and the drive of the de-rotating element with precisely half the rotational angular velocity, compared against the principal motion, require an expensive mechanical high-precision system. Moreover, the interposition of the de-rotating element requires long open light paths and high coupling losses involved. Specifically with the application of single-mode fibers an insufficient quality in optical transmission is achieved.
Among the transmission systems presented here, the two systems mentioned first must be ruled out for the majority of applications because they do not involve a sufficient bandwidth or because they permit only a unidirectional transmission. For this reason, the illustrated arrays with rotation compensation will preferably be discussed in the following description. Here, the nucleus consists in inverting optical elements that are rotated with half the angular velocity of the principal rotation. The light beams of the individual transmission channels may be inverted with an odd number of reflections by diffraction in a single axis only or by the shape of a pipe of optical conductors. The most important designs of an inverting optical element for these purposes, which are mentioned in literature, are as follows:                Dove prism        delta prism        Schmidt-Pechan prism        centrally mirrored sphere        inverting optical-fiber bundle (de-rotation plate)        axially mirrored gradient rod lens (slab lens).        
The considerations of the optical systems described herein are not only applicable to systems rotating relative to each other but also to systems displaceable along an axis. In linearly displaceable systems, a de-rotating element is, of course, not necessary.
The optical systems used for coupling and decoupling have a decisive influence on the quality of the transmission systems described here. Various designs of these optical systems in correspondence with prior art, specifically those envisaged for the application of the third transmission technology operating on de-rotating elements, will be explained below. Even though the discussion will focus on exemplary glass fibers these considerations apply, by way of analogy, to other forms of optical waveguides as well, such as synthetic light-conducting fibers, or to active components such as transmitters or receivers.
U.S. Pat. No. 4,725,116 discloses a modular multi-channel transmission system. There, each channel is transmitted by a separate mirror. The mechanical complexity of this system is substantial because a separate optical system and a corresponding precise supporting system are required for each mirror. Apart therefrom, the length of the optical paths increase as the number of channels increases, which, in its turn, means an impairment of the optical characteristics. Moreover, attenuation peaks occur at those sites where the feeding fibers pass the optical path. Furthermore, the overall length and the weight increase in proportion to the number of channels. As a consequence, the unit grows to a size that is unacceptable for many applications with a small number of channels already.
U.S. Pat. No. 4,872,737 discloses a multi-channel transmitter operating on the basis of a Dove prism. Beam coupling or decoupling is performed by several separate collimators. These collimators must be adjusted individually. A precise adjustment requires a comparatively long mechanical lever or a fine thread that requires additional space, too. As a result, the area to be projected, i.e. the entire surface projected via the de-rotating system, increases as the number of channels and the precision in adjustment increases. Therefore, a larger optical system is necessary which, in its turn, displays a higher optical attenuation as a result of the longer paths and, at the same time, involves higher demands on the precision in adjustment. For this reason, the enlargement of a mechanical lever in such an approach, for example, for an increase of the precision in adjustment does not result in an improvement. As the number of channels increase, rather improved adjustment systems must be found which involve a higher precision in adjustment at the same space requirement. Here the rhomboid prism mirrors for beam deviation or parallel beam shift, which are equally proposed in that above-mentioned patent, offer one possible solution. Even though the application of a larger-size de-rotation system is therefore avoided, the additional optical path and the additional surfaces entail an impairment of the optical transmission. The additional components increase the system costs.
An extremely flexible adjustment without a strong enlargement of the area to be projected is proposed in U.S. Pat. No. 5,157,745. The adjustment of the individual channels is performed here by correction lenses disposed for displacement in a direction orthogonal on the optical path, which can be used, via adjustment, to achieve the high precision required for single-mode transmission. The solution is very complex and expensive because a high number of components must be subjected to a troublesome time-consuming adjustment process. The additional adjustment components render the transmission system bulky, heavy and expensive. This system is certainly well-suited for stationary applications such as those in radar installations, but in mobile applications, which involve the occurrence of shocks and vibrations, the optical system will become misaligned very soon. Apart therefrom, due to the high number of air/glass transitions a worse quality in transmission will be achieved than in comparable systems without such transitions.
In an approach to avoid the aforedescribed disadvantages according to U.S. Pat. No. 5,442,721, a bundle of conventional collimators is mounted as a complete unit into the transmitter. As a consequence, only a single adjustment of the entire collimator bundle is still required. A substantial disadvantage of such bundling resides, however, in the fact that the rather substantial tolerances of the individual collimators cannot be compensated. The individual collimators as such are composed of fiber mounts and lens systems that are fixedly interconnected in a ferrule. The mounting precision of these collimators is limited and, as a rule, does not satisfy the demands on a rotary transmitter system. Such an approach, in which individual adjustment of the collimators is no longer possible, is therefore not applicable specifically in a single-mode transmission system.
All the aforedescribed solutions involve the disadvantage that a sufficient optical precision cannot be achieved on account of lack of adjustment potentials or that adjustment is highly complex and unreliable in continuous operation. Moreover, none of the aforedescribed solutions permits the realization of high channel numbers with more than 5 to 10 channels.